Lateral variations of seismic-wave attenuation in the upper mantle above the inclined earthquake zone of the Tonga Island Arc: Deep anomaly in the upper mantle

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Large variations of the predominant frequencies and amplitudes of the seismic phases Pn and Sn occur for hundreds of paths that cross much of the region between the Tonga island arc and the Fiji islands. The data clearly delineate a region of high attenuation in the uppermost mantle behind the Tonga arc. The region of high attenuation, a zone about 300 km wide, is between the Tonga ridge (the frontal arc) and the Lau ridge (the third arc). The alignment of active volcanoes on the Tonga ridge seems to mark the eastern boundary of this high-attenuation zone. If low attenuation, or high Q, correlates with strength, our observations indicate that the uppermost mantle is considerably weaker beneath the Lau basin, and a gap in or a substantial thinning of the lithosphere exists between the frontal arc and the third arc. This is in excellent agreement with the hypothesis that the Lau basin is the product of crustal extension. A detailed study of the variations of predominant frequencies of P and S waves that traverse the wedge of mantle above the inclined seismic zone of intermediate and deep earthquakes of Tonga shows that material with anomalously high attenuation is present throughout the prism of mantle above depths of about 150 to 300 km along the inclined seismic zone. That is, the data imply an anomalous increase in attenuation in the asthenosphere near the inclined seismic zone; Q may be as small as 50 for P waves and less than 20 for S waves. Regions of anomalously high attenuation also exist behind several other island arcs in a position similar to that behind the Tonga arc. These zones seem compatible with models in which the asthenosphere material is mobilized by the descending lithospheric slab.